Make Callback::Reset() return a copy to support use-cases where Run() ends up modifying |*this|.

base/callback.h.pump

Index: base/callback.h.pump
diff --git a/base/callback.h.pump b/base/callback.h.pump
index ef65e8322f21c2b969538b5148c12935f7492fde..57db9d03519c641a93e67cb31b32716b988f26e1 100644
--- a/base/callback.h.pump
+++ b/base/callback.h.pump
@@ -44,6 +44,10 @@ $var MAX_ARITY = 7
 // The reason to pass via a const-reference is to avoid unnecessary
 // AddRef/Release pairs to the internal state.
 //
+// If a Callback variable must be empty during the Run() of a previous value of
+// that variable, use ResetAndRun().  E.g. if MyClass has a member variable
+// Callback which gets set as a side-effect of Run()'ing the Callback (to
+// something else), ResetAndRun() can be handy.
 //
 // EXAMPLE USAGE:
 //
@@ -292,6 +296,16 @@ $for ARG ,
              [[internal::CallbackForward(a$(ARG))]]);
   }
 
+
+  R ResetAndRun($for ARG ,
+        [[typename internal::CallbackParamTraits<A$(ARG)>::ForwardType a$(ARG)]]) {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run(
+$for ARG ,
+            [[internal::CallbackForward(a$(ARG))]]);
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*[[]]